Dialkylidenecyclobutane/bisimide/epoxy compositions

ABSTRACT

A composition comprising a 1,2-dialkylidenecyclobutane such as 1,2-dimethylenecyclobutane, a polyimide such as a bismaleimide, an epoxy resin and a curing agent for the epoxy resin can be thermally cured to a tough polymer blend having a high glass transition temperature.

BACKGROUND OF THE INVENTION

This invention relates to thermosettable resin compositions. In one embodiment, the invention relates to enhancement of the processing properties of bismaleimides.

Advanced composites are high-performance materials made up of a fiber-reinforced thermoplastic or thermosettable material. Thermosettable materials useful in advanced composites applications must meet a set of demanding property requirements. For example, such materials optimally have good high-temperature properties such as high (above 200° C.) cured glass transition temperature and good mechanical strength. For ease of processing in preparing prepregs for composite parts, the uncured material will ideally have a low (below 120° C.) melting temperature and a wide temperature range of processable viscosity (a wide "processing window").

Bismaleimide resins have superior high-temperature properties but are very brittle and further tend, because of their high softening points, to require solvents in order to be readily processable. In addition, standard cured bismaleimide resins tend to have high (in the 5-7% range) 93° C. water absorption. Addition of thermoplastic or cyanate-terminated oligomers to bismaleimides increases the toughness but produces uncured mixtures so high in viscosity that fiber impregnation and processing by standard thermoset techniques are difficult.

It is thus an object of the invention to provide new thermoset resin materials. In one aspect, it is an object of the invention to provide curable additives which reduce the melting and softening points of bismaleimides yet cure to high-Tg, tough resins.

SUMMARY OF THE INVENTION

According to the invention, a composition is provided comprising a bisimide, a 1,2-dialkylidenecyclobutane, an epoxy resin and a curing agent for the epoxy resin. The invention blends can be melt-processed for composites applications and have good toughness in the cured state.

DETAILED DESCRIPTION OF THE INVENTION

The invention composition includes a bisimide of an unsaturated dicarboxylic acid. The preferred bisimides are N,N'-bisimides of unsaturated dicarboxylic acids which can be represented by the formula ##STR1## in which Y is a substituted or unsubstituted divalent group containing at least 2 carbon atoms, preferably 2 to 6 carbon atoms, and a carbon-carbon double bond, and Z is a divalent group containing at least 1 and generally about 1 to 40 carbon atoms. Z can be aliphatic, cycloaliphatic, aromatic or heterocyclic. A preferred class of bisimides comprises bismaleimides derived from aromatic amines and can be represented by the formula ##STR2## in which each R₁ is selected independently from H, C₁₋₂ alkyl or halide; R₂ is selected from divalent hydrocarbon radicals containing from about 1 to about 10 carbon atoms, --O--, --SO₂ --, --COO--, --CONH--, --CO-- and --S--; and each R₃ is selected independently from H, C₁₋₃ alkyl and halide. The aromatic rings may alternatively be heterocyclic.

Examples of such bisimides include

1,2-bismaleimidoethane;

1,6-bismaleimidohexane;

1,3-bismaleimidobenzene;

1,4-bismaleimidobenzene;

2,4-bismaleimidotoluene;

4,4'-bismaleimidodiphenylmethane;

4,4'-bismaleimidodiphenylether;

3,3'-bismaleimidodiphenylsulfone;

4,4'-bismaleimidodiphenylsulfone;

4,4'-bismaleimidodicyclohexylmethane;

3,5-bis(4-maleimidophenyl)pyridine;

2,6-bismaleimidopyridine;

1,3-bis(maleimidomethyl)cyclohexane;

1,3-bis(maleimidomethyl)benzene;

1,1-bis(4-maleimidophenyl)cyclohexane;

1,3-bis(dichloromaleimido)benzene;

4,4'-biscitraconimidodiphenylmethane;

2,2-bis(bis(4-maleimidophenyl)propane;

1-phenyl-1,1-bis(4-maleimidophenyl)ethane;

α,α-bis(4-maleimidophenyl)toluene,

3,5-bismaleimido-1,2,4-triazole,

and various N,N'-bismaleimides disclosed in U.S. Pat. Nos. 3,562,223, 4,211,860 and 4,211,861. Bismaleimides can be prepared by methods known in the art, as described in U.S. Pat. No. 3,018,290, for example.

The bisimide resin can contain imide oligomers according to the formula ##STR3## in which x is a number within the range of about 0 to about 3. Such oligomers may be present as an impurity in difunctional bisimides.

The preferred bisimide resin in 4,4'-bismaleimidodiphenylmethane. Suitable N,N'-unsaturated bismaleimide resins are commercially available from Technochemi GmbH as Compimide® resins, for example.

The invention composition includes a 1,2-dialkylidenecyclobutane, including those which can be described by the structural formula ##STR4## in which each R is selected independently from hydrogen, C₁₋₁₀ alkyl, halo, aryl, alkoxy, aryloxy, alkylthio, arylthio and dialkylamino. The presently preferred 1,2-dialkylidenecyclobutane, because of the properties of a bisimide copolymer prepared therewith, is 1,2-dimethylenecyclobutane, which is defined by the above formula when each R is hydrogen.

In general, dialkylidenecyclobutanes can be prepared by the thermal dimerization of the corresponding allenes in a recirculating hot-tube reactor. Specifically, the process will generally be carried out by circulating a stream of gaseous allene through a tube reactor at 450°-600° C. with a residence time in the hot zone of 0.1 to 10 seconds. Downstream from the hot zone, the stream is cooled sufficiently to condense the dialkylidenecyclobutane. Unchanged allene (combined with a fresh makeup stream) is recirculated back to the hot zone by a pump. Such a process is described for 1,2-dimethylenecyclobutane in Chernykh et al., Neftepererab. Neftekhim., 1981 (7), 48-50. Synthesis of 1,2-dimethylenecyclobutane is also illustrated in Example 1 herein. The allene starting material can be produced by pyrolysis of isobutylene or by isolation from a hydrocarbon mixture such as a refinery cracker stream.

The invention composition includes an epoxy resin. Suitable epoxy resins include glycidyl ethers prepared by the reaction of epichlorohydrin with a compound containing an average of at least about two hydroxyl groups carried out under alkaline reaction conditions. The epoxy resins obtained when the hydroxyl group-containing compound is bisphenol-A are represented below ##STR5## in which n is a number within the range of 0 to about 2. Epoxy resins suitable for the invention compositions have molecular weights generally within the range of 100 to about 2000, preferably about 200 to about 1500. The commercially-available liquid epoxy resin EPON® Resin 828, a reaction product of epichlorohydrin and 2,2-bis(4-hydroxyphenyl)propane (bisphenol-A) having a molecular weight of about 400, an epoxide equivalent (ASTM D-1652) of about 185-192 and an n value in the formula above of about 0.2, is presently preferred because of its low viscosity and commercial availability.

The composition will preferably include a curing agent for the epoxy resin which in most cases will be essentially unreactive with maleimide or diene groups. Acceptable curing agents include acid anhydrides, bisphenols, polyphenols, multifunctional carboxylic acids, dicyandiamide, cyanamide, substituted cyanamides, biscyanamides, active methylene compounds such as cyanoacetates and acetoacetates, and cationic agents such as BF₃ complexes.

The polyimide, triene and epoxy monomers may be combined in any manner desired, such as melt, solution or powder blending. The preferred technique, when sufficiently large quantities of monomers are used, involves heating a mixture of the solid polyimide, the liquid 1,2-dialkylidenecyclobutane and the epoxy resin with stirring at a temperature above the respective melting points but below the polymerization temperature of any monomer, until the mixture becomes a homogeneous melt. The melt may optionally be held at temperatures above about 120° C. for desired periods of time in a process of prepolymerization to increase the crystallization resistance of the melt and/or to increase its viscosity to desired levels. The mixture can then be poured directly into a mold for polymerization, or it can be cooled for later polymerization. For small quantities of monomers, however, excessive amounts of the dialkylidenecyclobutane may volatilize during the melt reaction, upsetting the desired stoichiometric balance. In these cases, it is preferable for the monomer mixture to be processed in a two-step process in which the monomer mixture is reacted in a solvent, with the solvent then evaporated and the adduct melted and cured to solid polymer without solvent.

The relative amounts of the monomers will depend upon the cured and uncured properties desired. In general, optimum properties will be achieved with a dialkylidenecyclobutane:bisimide molar ratio within the range of about 0.5:1 to about 2:1, preferably about 0.8:1 to about 1.5:1, and a weight ratio of (epoxy resin+epoxy curing agent)/(dialkylidenecyclobutane+bisimide) within the range of about 0.05:1 to about 10:1, preferably about 0.1:1 to about 1:1. The optimum ratio of curing agent to epoxy will vary with the type of curing agent used.

The composition may contain an optional free radical inhibitor to inhibit free radical polymerization of the bisimide monomer. Generally, the free radical inhibitor will be present in the composition in an amount within the range of about 0.0002 to about 0.02 moles per mole of the bisimide, preferably from about 0.001 to about 0.01 moles. The free radical inhibitor can be added to the monomers in any manner effective for intimate blending of the monomers and free radical inhibitor. Free radical inhibitors include phenols such as t-butylcatechol, hydroquinone and p-methoxyphenol; quinones such as 1,4-benzoquinone and 1,4-naphthoquinone; polynitro aromatics such as picric acid and 2,4,6-trinitrotoluene; hydroxylamines such as diethylhydroxylamine; stable radicals such as di-t-butylnitroxide or diphenylpicrylhydrazyl; and certain polycyclic heterocycles such as phenothiazine. The preferred free radical inhibitor is phenothiazine.

Polymerization is effected by heating the mixture to a temperature effective to initiate opening of the cyclobutene ring (formed by the initial Diels-Alder reaction of the diene group of the dialkylidenecyclobutane with the dienophilic double bond) to form a transient diene which rapidly reacts with available maleimide groups. This temperature is generally at least about 150° C., preferably about 180° to about 350° C., held for a time of about 0.5 hour or more (with the required cure time dependent on the temperature-staging program used).

In order to achieve optimum properties, a mixture of the monomers and free radical inhibitor is heated at a temperature near or above the ultimate (fully-cured) glass transition temperature of the polymer blend for a time sufficient to achieve essentially complete reaction of the monomers. "Essentially complete" reaction of the monomers has been reached when no further reaction exotherm is observed by differential scanning calorimetry (DSC) upon heating the cured polymer blend. The time of the heat treatment, or "post-cure," will vary depending upon the monomers, the degree of pressure applied and any pre-curing of the monomer mixture. Preferably, this post-cure is at or above the ultimate Tg, but will always be at a temperature lower than the temperature at which degradation of the polymer product will occur at significant rates.

The product polymer blends are useful in adhesives, coatings and as resin matrices for composites in aerospace and electronics applications, including large structural parts and circuit boards. Based on their long shelf life and relatively low melting point, some of the uncured mixtures are useful for making tacky prepregs which can then be molded into composites. They are also suitable for low-solvent or solventless liquid resin processing methods such as filament winding, resin transfer molding and pultrusion if the mixtures are heated to provide sufficiently low viscosity for fiber impregnation.

Electrical applications for the invention compositions include encapsulation of electronic devices and electrical lamination for circuit board manufacture. In encapsulation, the composition will usually be combined, generally by melt-blending, with a suitable inert filler such as particulate silica. For lamination, the composition will be applied, in organic solution or in a solventless melt, to a suitable reinforcement such as glass fiber, and partially cured to form an electrical prepreg, which will subsequently be fabricated into a fully-cured laminate.

For preparation of reinforced laminate materials, a fibrous substrate of glass, carbon, quartz, poly(p-phenyleneterephthalamide), polyester, polytetrafluoroethylene, poly(p-phenylenebenzobisthiazole), boron, paper or like material, in chopped, mat or woven form, is impregnated with a bisimide/dialkylidenecyclobutane/epoxy resin/epoxy curing agent composition in molten or solution form. A prepreg is formed by heating the impregnated substrate in an oven at a temperature sufficient to remove the solvent and to partially cure without gelation, or "B-stage," the resin system, generally about 120° C. to about 180° C., preferably about 135° to about 175° C., for a time of up to about 2 hours, preferably about 10 to about 40 minutes. A laminate is fabricated by subjecting a set of layered prepregs to conditions effective to cure the resins and to integrate the prepregs into a laminated structure. The laminate can optionally include one or more layers of a conductive material such as copper.

Laminating generally involves subjecting the prepregs to a temperature above about 175° C., preferably from about 180° to about 350° C. for a time of at least about 10 minutes, at a pressure within the range of about 50 to about 500 psi.

For some laminating applications, it may be advantageous to heat treat, or upstage, the monomer mixture prior to application to a laminating substrate, particularly if the mixture will be stored prior to use. Suitable heat treatment involves subjecting the mixture to an elevated temperature for a time sufficient to cause sufficient reaction and viscosity increase to inhibit crystallization of either or both monomers from the mixture upon storage, but not sufficient to gel the composition. Such heat treatment conditions generally include a temperature of at least about 120° C., preferably about 135° to about 175° C., for a time of at least about 10 minutes, preferably about 12 to about 90 minutes. The resulting mixture will be less tacky and less susceptible to crystallization of the components with storage.

EXAMPLE 1

Preparation of 1,2-Dimethylenecyclobutane. A recirculating apparatus for the thermal dimerization of allene was designed as follows. The heated reactor was a bank of approximately 110 segments (each about 30 cm long) of stainless steel tubing 1.27 cm in outside diameter. The segments were arranged vertically in series and connected to one another by U-shaped stainless steel connectors to which they were welded. The volume of the heated portion of the reactor was about 3.4 liters. The bank of tubes was immersed in a fluidized bed of aluminum oxide particles. Thermocouples wedged between the connectors of the reactor at various points allowed one to monitor the wall temperature of different segments of the reactor.

Downstream from the reactor was a cold trap containing a cooling fluid at approximately -65° C. above a flask which functioned as a gas-liquid separator. Downstream from the first trap was a second trap filled with dry ice in dichloromethane, guarding the outlet to the system (through an oil bubbler) to condense any allene which otherwise could have escaped from the system. Condensed allene from this second trap fell into the gas-liquid separator. The condensed material (allene dimers and some of the allene) from the traps fell to the bottom of the separator and then flowed through a fluoropolymer tube into a reservoir for liquid allene and allene dimers. Sufficient heat was applied to this reservoir to keep the allene boiling gently. The allene not condensed by the cold traps was combined with that evaporating from the reservoir. This stream of recovered allene was passed through a filter into a diaphragm pump which recirculated the allene back into the hot tube. A makeup stream of fresh allene from a cylinder was also introduced into the loop just upstream from the recirculation pump.

The system was first purged with nitrogen. The power to the fluidized bed was turned on and its temperature was brought to 450°-470° C. Allene was introduced into the system from the allene cylinder at a rate of 80-100 g/hr. The allene supply from the cylinder was shut off two to three hours before the end of a dimerization run in order that the allene present in the system could be used up, with little allene remaining in the reservoir at the end. At the end of the day, the power to the fluidized bed was turned off, the system was allowed to cool, and the accumulated dimer was poured into a bottle and weighed. Approximately 3 g of phenothiazine was added per kilogram of dimer to inhibit polymerization of the 1,2-dimethylenecyclobutane. The crude dimer was then analyzed by gas chromatography for peaks corresponding to two allene dimers, 1,2-dimethylenecyclobutane (1,2-DMCB) and 1,3-dimethylenecyclobutane (1,3-DMCB), and a component shown by mass spectrometry to have a molecular formula of C₉ H₁₂ (an allene trimer). Data from seven hot tube reaction runs are shown in Table 1.

                                      TABLE 1                                      __________________________________________________________________________                                    GC analysis                                     Reaction                                                                             Reaction                                                                             Allene                                                                              Crude dimer                                                                            Crude 1,3-DMCB,                                                                             1,2-DMCB,                                                                             C.sub.9 H.sub.12                  #     time, hr.                                                                            used, g                                                                             produced, g                                                                            yield, %                                                                             %      %      peak, %                           __________________________________________________________________________     1      8.0  558  443     79.4  8.4    67.0   15.0                              2     15.8  1197 881     73.6  8.1    75.0   11.0                              3     11.3  862  753     87.3  8.3    73.4   11.4                              4     11.2  824  647     78.5  8.3    71.6   14.0                              5     11.8  932  806     86.5  8.5    68.7   15.4                              6     11.4  909  746     82.1  8.4    68.0   16.2                              7     11.0  872  724     83.0  8.5    69.0   15.7                              __________________________________________________________________________

The products of the seven runs in Table 1 were flash-distilled under vacuum to remove tars and were subsequently distilled under reduced pressure in 2.54 cm Oldershaw columns with 30 plates. The resulting distilled fractions and similarly-obtained DMCB cuts were used in the following examples.

Example 2

Cure of Epoxy Resin with Mixtures of a Bismaleimide and 1,2-Dimethylenecyclobutane (DMCB). Ten mixtures (mixtures #1-10 in Table 2 below) were prepared as follows. Mixtures #9 and #10 were control DMCB-BMI blends containing no epoxy component. In mixture #9 the BMI was COMPIMIDE® MDAB (bismaleimide of 4,4'-methylenedianiline), while in mixture #10 the BMI was COMPIMIDE 353A (a commercial bismaleimide mixture). In mixtures #1 and #2, the DMCB-BMI system based on COMPIMIDE 353A was diluted with EPON® Resin 828, with Nadic methyl anhydride (NMA), predominantly a maleic anhydride adduct of methylcyclopentadiene, added as curing agent. In mixtures #3 and #4 the bismaleimide was COMPIMIDE MDAB and the epoxy curing agent was methyl hexahydrophthalic anhydride (MHHPA). In mixtures #5 through #8, the epoxy system was EPON® Resin 825 cured with bisphenol A (with amounts of epoxy system and the accelerator used being varied).

In preparation of the mixtures, to 250- or 500-mL glass bottles were added the amounts shown of bismaleimide, a distilled dimethylenecyclobutane (DMCB) fraction containing mostly 1,2-isomer, phenothiazine (to inhibit radical polymerization of the diene and maleimide groups) and, when used, Monsanto PC-1344 defoamer (an acrylic oligomer, added to prevent excessive foaming during vacuum degassing and to enable the preparation of void-free castings), along with the indicated amount of dichloromethane as solvent. The bottles were placed on rollers and rolled overnight (or longer) at room temperature to allow completion of the first-stage Diels-Alder reaction between the 1,2-dimethylenecyclobutane and the maleimide groups of the bismaleimide. The epoxy resin, curing agent (anhydride or bisphenol) and DABCO T-45 (a glycol solution of potassium 2-ethylhexanoate, an accelerator of epoxy cure with anhydrides or bisphenols) or phenyldimethylurea (PDMU) (also an epoxy cure accelerator) were added to the mixtures at this point and the blends were mixed until homogeneous.

The mixtures were then poured into 125-mL or 250-mL Erlenmeyer flasks with a vacuum connection. The flasks were then placed in a 120° C. oil bath (150° C. for mixtures #9 and #10 because of their higher processing temperature requirement) and the contents were swirled as dichloromethane, 1,3-dimethylenecyclobutane, and other volatile unreacted materials were removed, first at atmospheric pressure and then under mechanical pump vacuum for a few minutes until bubbling had essentially stopped.

Degassed molten mixtures #1-#8 were then poured into a two-piece rectangular stainless steel mold, preheated to 120° C., with a 1/8" (1.59 mm) thick cavity, with the mold parts separated by a gastight silicone rubber gasket such that the mold could be pressurized during cure. These mixtures were cured for 1 hour at 120° C., followed by ramping linearly to 260° C. over a period of 3.5 hours and then holding for 1 hour at 260° C. A few grams of each uncured sample were kept as a retain for characterization of uncured properties. Control mixtures #9 and #10, containing no epoxy component, were cured in a mold similar to the above but the cure schedule was 1 hour at 150° C., followed by ramping linearly to 290° C. over a period of 3.5 hours and then holding for 1 hour at 290° C. Uncured and cured properties of the materials are shown in Table 2.

One can see from Table 2 that the epoxy-diluted systems all reached a 1 Pa.s viscosity (ideal for fiber impregnation) at a considerably lower temperature than did the unmodified DMCB-BMI systems (experiments #9 and #10). In most cases, dynamic mechanical Tg (as measured by Rheometrics tan δ peak) decreased by only a small amount as a result of epoxy modification. Modification with epoxy cured with Nadic methyl anhydride (NMA) decreased toughness substantially, but loss of toughness was much less for systems modified with epoxy cured with methyl hexahydrophthalic anhydride (MHHPA). Although there was significant loss of toughness for systems modified with epoxy cured with bisphenol A, it was noteworthy that methylene chloride absorption dropped to very low values for a number of the blends with bisphenol-A cured epoxy (and also room temperature dry flexural modulus was considerably higher than for the DMCB-BMI systems without epoxy modification).

    TABLE 2       Experiment # 1 2 3 4 5 6 7 8 9 10        Composition:           COMPIMIDE ® MDAB, grams   78.84 78.84 39.42      59.13 39.42 59.13 73.10 COMPIMIDE 353A, grams 80 80        80 Total      bismaleimides, moles 0.2433 0.2433 0.2200 0.2200 0.1100 0.1650 0.1100      0.1650 0.2040 0.2433 1,2-Dimethylenecyclobutane (DMCB): Crude distillate,       grams 21.60 21.37 19.10 19.10 9.55 14.33 9.48 14.23 19.78 23.60 %      1,2-isomer in crude distillate (GC area) 90.204 91.222 92.281 92.281      92.281 92.281 92.943 92.943 82.606 82.606 Net 1,2-isomer, grams 19.48      19.49 17.63 17.63 8.81 13.22 8.81 13.23 16.34 19.50 moles 0.2432 0.2433      0.2200 0.2200 0.1100 0.1650 0.1100 0.1651 0.2039 0.2433 Phenothiazine,      grams 0.23 0.23 0.2367 0.2367 0.1037 0.1542 0.1058 0.1567 0.21 0.23      Monsanto PC-1344 defoamer, grams 0.26 0.26 0.26 0.26 0.1084 0.1592      0.1071 0.1592 0.24 0.26 CH.sub.2 Cl.sub.2, grams (solvent) 180 180 180      180 90 135 90 135 180 180 EPON ® Resin 828, grams 10.0 20.0 10.0      20.0 EPON Resin 825, grams     32.50 16.25 32.50 16.25 NMA (96%), grams      9.0 18.0 MHHPA, grams   8.10 16.20 Bisphenol A, grams     20.0 10.0 20.0      10.0 PDMU, grams     0.3242 0.1621 DABCO T-45, grams 0.2016 0.3983      0.1976 0.4042   0.3289 0.1622 Uncured Properties: Temp., °C., at      which viscosity is 1 Pa.s 107 94 93.sup.a 109.sup.a 87 104 88 108 125      124 Cured Properties: Rheometrics tan δ peak, °C. 233 223      260 255 92,325 285 90 281 291 264 R.T. dry flexural (ASTM D-790):      Yield/break (y/b) strength, MPa 111 ± 11(b) 97 ± 12(b) 111 ±      32(y) 111 ± 18(b) 103 ± 25(b) 115 ± 25(b) 117 ± 11(b) 128      ± 28(b) 122 ± 1(y) 139 ± 1(y) Tangent modulus, GPa 3.29 ±      0.03 3.44 ± 0.02 2.97 ± 0.06 2.99 ± 0.04 3.43 ± 0.04 3.69      ± 0.02 3.31 ± 0.02 3.62 ± 0.03 2.67 ± 0.02 3.12 ± 0.04      Break elongation, % 3.6 ± 0.5 2.9 ± 0.5 >4.7 4.2 ± 1.1 3.2 ±      0.9 3.2 ± 0.7 3.9 ± 0.5 3.9 ± 1.2 >6.5 >6.5 93° C. wet      flexural (ASTM D-790): Yield/break (y/b) strength, MPa 70 ± 3(b) 66      ± 5(b) 74 ± 6(b) 70 ± 11(b) 34 ± 1(y) 94 ± 17(b) 34 ±      1(y) 99 ± 25(b) 81 ± 1(y) 74 ± 1(b) Tangent modulus, GPa 2.67      ± 0.03 2.71 ± 0.05 2.41 ± 0.02 2.38 ± 0.03 0.81 ± 0.01      3.02 ± 0.03 0.76 ± 0.01 2.89 ± 0.05 2.33 ± 0.02 2.62 ±      0.04 Break elongation, % 3.0 ± 0.2 2.6 ± 0.3 3.8 ± 0.5 3.6 ±      0.9 >6.5 3.6 ± 0.9 >6.5 4.2 ± 1.6 >6.5 6.9 ± 0.5 Compact      tension fracture toughness, Kq, 0.99 ± 0.01 0.63 ± 0.07 1.93 ±      0.04 1.42 ± 0.06 0.70 ± 0.14 0.83 ± 0.01 1.17 ± 0.03 0.84      ± 0.05 2.93 ±  0.13 2.47 ± 0.29 MPa-m.sup.1/2  (ASTM E 399-83)      Dielectric constant, 1 MHz (ASTM D229/15) 3.04 3.16 3.39 3.35 3.43 3.20      3.59 3.45 3.40 3.20 Dissipation factor, 1 MHz (ASTM D229/15) 0.0145      0.0160 0.0117 0.0123 0.0204 0.0135 0.0268 0.0708 0.0156 0.0143 93°       C. H.sub.2 O pickup, %: 1 day 1.59 1.41 1.19 1.11 1.89 1.42 1.55 0.98      1.66 2.25 2 weeks 2.62 2.46 1.78 1.62 3.39 2.17 3.01 1.63 2.28 3.60 Room      temp. methyl ethyl ketone pickup, %: 1 day 0 0 0 0 0.05 0 0.10 0.15 0 0      2 weeks 0 0 0 0 8.75 0 6.16 0 1.69 0 Room temp. CH.sub.2 Cl.sub.2      pickup, %: 1 day disint. disint. disint. disint. 37.2 0 37.2 0 169 249 2      weeks disint. disint.   62.0 0.63 63.3 1.35 179 disint.      Appears likely that these samples may have been reversed 

I claim:
 1. A composition comprising(a) a difunctional bisimide of an unsaturated dicarboxylic acid as represented by the structural formula ##STR6## in which Y is a substituted or unsubstituted divalent moiety containing at least 2 carbon atoms and a carbon-carbon double bond, and Z is a divalent linking group; (b) an epoxy resin; (c) a 1,2-dialkylidenecyclobutane as represented by the formula ##STR7## in which each R is selected independently from the group consisting of hydrogen, C₁₋₁₀ alkyl, halo, aryl, alkoxy, aryloxy, alkylthio, arylthio and dialkylamino; (d) a curing agent for the epoxy resin.
 2. The composition of claim 1 in which the epoxy resin is a diglycidyl ether of a dihydric phenol.
 3. The composition of claim 2 in which component (a) is a bismaleimide.
 4. The composition of claim 3 in which component (c) is 1,2-dimethylenecyclobutane.
 5. The composition of claim 4 in which the weight ratio of (epoxy resin+epoxy curing agent)/(dialkylidenecyclobutane+bisimide) is within the range of about 0.05:1 to about 10:1.
 6. The composition of claim 1 in which component (a) comprises 4,4'-bismaleimidodiphenylmethane and component (c) is 1,2-dimethylenecyclobutane.
 7. The composition of claim 3 which further comprises an effective amount of a free radical inhibitor for the bismaleimide.
 8. The composition of claim 1 which further comprises a fibrous reinforcing agent.
 9. The composition of claim 1 which further comprises particulate silica.
 10. A polymeric composition comprising the product of contacting, at a temperature of at least about 180° C., monomers comprising(a) a bisimide of an unsaturated dicarboxylic acid as represented by the structural formula ##STR8## in which Y is a substituted or unsubstituted divalent moiety containing at least 2 carbon atoms and a carbon-carbon double bond, and Z is a divalent linking group; (b) an epoxy resin; (c) a 1,2-dialkylidenecyclobutane as represented by the structural formula ##STR9## in which each R is selected independently from the group consisting of hydrogen, C₁₋₁₀ alkyl, halo, aryl, alkoxy, aryloxy, alkylthio, arylthio and dialkylamino (d) a curing agent for the epoxy resin.
 11. The composition of claim 10 in which the weight ratio of (epoxy resin+epoxy curing agent)/dialkylidenecyclobutane+bisimide) is within the range of about 0.05:1 to about 10:1.
 12. The composition of claim 10 in which the epoxy resin is a diglycidyl ether of a bisphenol.
 13. The composition of claim 10 in which component (a) is a bismaleimide and component (c) is 1,2-dimethylenecyclobutane.
 14. The composition of claim 13 in which the epoxy resin is a liquid diglycidyl ether of bisphenol-A.
 15. A prepreg comprising the composition of claim 1 and a fibrous substrate.
 16. A process for preparing a copolymer comprising the steps of(a) preparing a mixture of a difunctional bisimide of an unsaturated dicarboxylic acid, an epoxy resin, a curing agent for the epoxy resin and from about 0.5 to about 2.0 moles, per mole of the difunctional bisimide, of a 1,2-dialkylidenecyclobutane as represented by the structural formula ##STR10## in which each R is selected independently from the group consisting of hydrogen, C₁₋₁₀ alkyl, halo, aryl, alkoxy, aryloxy, alkylthio, arylthio and dialkylamino; and (b) heating said mixture to a temperature within the range of about 180° to about 350° C. for at least about 0.5 hour.
 17. The process of claim 16 in which the difunctional bisimide comprises 4,4'-bismaleimidodiphenylmethane.
 18. The process of claim 17 in which the difunctional bisimide further comprises at least one of 2,4-bismaleimidotoluene or 1,3-bismaleimidobenzene.
 19. The process of claim 16 in which said mixture further comprises phenothiazine. 